This invention relates to light-beam deflection instruments, such as refractometers, in which an electrical output is generated that is dependent upon the difference between two optical measurements.
In one class of differential refractometers, light is passed through two fluids in a measurement cell, and electrical outputs related to the apparent relative refractivities of the fluids are generated. The electrical outputs are compared, e.g., by measuring their difference. Such measurement systems can be "zeroed" by adjusting the optics at the start of each measurement (e.g., so as to make the two electrical outputs equal), and further by including in the system output an adjustable offset voltage. However, changes in light intensity during the measurement cycle can throw off the accuracy of such zeroing.